Re: [PATCH 3/3] mm/mremap: check map count under mmap write lock and abstract

[Lorenzo Stoakes (Oracle)]

On Fri, Mar 27, 2026 at 09:22:31AM +0000, Pedro Falcato wrote:
> On Wed, Mar 11, 2026 at 05:24:38PM +0000, Lorenzo Stoakes (Oracle) wrote:
> > We are checking the mmap count in check_mremap_params(), prior to obtaining
> > an mmap write lock, which means that accesses to current->mm->map_count
> > might race with this field being updated.
> >
> > Resolve this by only checking this field after the mmap write lock is held.
> >
> > Additionally, abstract this check into a helper function with extensive
> > ASCII documentation of what's going on.
> >
> > Reported-by: Jianzhou Zhao <luckd0g@xxxxxxx>
> > Closes: https://lore.kernel.org/all/1a7d4c26.6b46.19cdbe7eaf0.Coremail.luckd0g@xxxxxxx/
> > Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@xxxxxxxxxx>
>
> Reviewed-by: Pedro Falcato <pfalcato@xxxxxxx>

Thanks!

>
> Shouldn't this have a Fixes: and go to stable?

Nah this is really hard to hit and doing the check is just making it so mremap
can bail out early, so it's not worth it.

>
> > ---
> >  mm/mremap.c | 88 +++++++++++++++++++++++++++++++++++++++++++++--------
> >  1 file changed, 75 insertions(+), 13 deletions(-)
> >
> > diff --git a/mm/mremap.c b/mm/mremap.c
> > index ba6c690f6c1b..ee46bbb031e6 100644
> > --- a/mm/mremap.c
> > +++ b/mm/mremap.c
> > @@ -1028,6 +1028,75 @@ static void vrm_stat_account(struct vma_remap_struct *vrm,
> >  		mm->locked_vm += pages;
> >  }
> >
> > +static bool __check_map_count_against_split(struct mm_struct *mm,
> > +					    bool before_unmaps)
> > +{
> > +	const int sys_map_count = get_sysctl_max_map_count();
> > +	int map_count = mm->map_count;
> > +
> > +	mmap_assert_write_locked(mm);
> > +
> > +	/*
> > +	 * At the point of shrinking the VMA, if new_len < old_len, we unmap
> > +	 * thusly in the worst case:
> > +	 *
> > +	 *              old_addr+old_len                    old_addr+old_len
> > +	 * |---------------.----.---------|    |---------------|    |---------|
> > +	 * |               .    .         | -> |      +1       | -1 |   +1    |
> > +	 * |---------------.----.---------|    |---------------|    |---------|
> > +	 *        old_addr+new_len                     old_addr+new_len
> > +	 *
> > +	 * At the point of removing the portion of an existing VMA to make space
> > +	 * for the moved VMA if MREMAP_FIXED, we unmap thusly in the worst case:
> > +	 *
> > +	 *   new_addr   new_addr+new_len         new_addr   new_addr+new_len
> > +	 * |----.---------------.---------|    |----|               |---------|
> > +	 * |    .               .         | -> | +1 |      -1       |   +1    |
> > +	 * |----.---------------.---------|    |----|               |---------|
> > +	 *
> > +	 * Therefore, before we consider the move anything, we have to account
> > +	 * for 2 additional VMAs possibly being created upon these unmappings.
> > +	 */
> > +	if (before_unmaps)
> > +		map_count += 2;
>
> oooh, shiny shiny diagrams.

:)

>
> --
> Pedro

Cheers, Lorenzo